Every two years or so, Mars' orbit brings it millions of miles closer to Earth than usual. This is an ideal time to send missions toward Mars — be they robots or future crews of astronauts.

Generally, if Mars is closer, missions are more efficient: Probes spend less time in transit, they need less fuel and they cost less overall. Everything from Odyssey in 2001 to Insight in 2018 took advantage of this window.

Now, some lawmakers are anxious to put manned Mars missions on NASA's calendar, and a close approach could be more than just efficient — it could be the only way to get astronauts there or back safely.

NASA limits its astronauts to a certain lifetime level of radiation exposure. Based on current mission designs, a round trip — just getting from A to B and back — could expose astronauts to more than those limits.

That's because most of this radiation comes from cosmic rays, and scientists still haven't come up with a reliable way to shield astronauts from their effects when they leave Earth's atmosphere. For now, the safest thing to do is to limit their exposure as much as possible — which would mean making as fast a trip as possible.

That said, current NASA policies are focused more on the moon than Mars, so it's not clear when a manned mission might happen. But the next close approach in 2020 will see lots of robot travelers: NASA, the European Space Agency, China and the United Arab Emirates all plan to launch unmanned missions.